The present invention relates to devices which transform the characteristic impedance of signal conductors and particularly transformers for high frequency signal transmission lines.
Transmission lines are frequently used to transmit high frequency electronic signals. In general, a transmission line comprises at least two parallel electrical conductors. The shape, size, spacing, and material of the conductors as well as the electrical attributes of any material used to insulate the conductors determine the characteristic impedance of the transmission line.
Generally, the impedance of a transmission line is selected to match that of the units which the line is connecting. However, not all units have the same characteristic impedance. If an output unit oof one impedance is connected by a transmission line to an input unit of a second impedance without a transformation of the impedance, signal reflections may arise at the ports of both the input and output units. The signal reflections caused by the mismatched impedance may cause signal distortion, ghost signals and/or noise and may cause a decrease in the efficiency of the signal transmission.
To avoid the problem caused by mismatched impedance, it is known to utilize a transformer at some point along the transmission line. The transformer is constructed to provide the correct impedance ratio between the output unit and the input unit.
A variety of impedance transformation devices for high frequency signal transmission lines are known. The U.S. Pat. No. 3,370,257 to Spierling discloses a transformer comprising a plurality of coaxial transmission lines, wherein each line is wound for a plurality of turns around a ferrite toroid. The lines are connected to each other i parallel at their input ends and in series at their output ends to provide an impedance transformation ratio of n.sup.2, where n is the number of lines in the transformer.
Another type of high frequency transformer is disclosed in U.S. Pat. No. 4,222,016 to Stock et al in which the transformer is comprised of a pair of identical sections of coaxial cable, each coaxial cable having an internal an an external conductor. The two sections have a length of approximately one-quarter of the operating wavelength of the signal transmitted and are disposed in parallel to one another. If the two sections are connected so that the internal conductor of the input end of a first one of the sections is connected to the external conductor of the input end of the second section, if the external conductor of the output end of the first section is connected to the external conductor of the output end of the second section, and if the unconnected conductors serve as terminals; the two sections will perform as a high frequency transformer with a transformation ratio of 1:1.
Other transformation ratios are also possible using such a transformer by providing two or more pairs of coaxial conductor sections, with the conductors of each pair connected to each other as described above and each pair connected to the other pairs so that one end is parallel connected and the other end is connected in series. In such transformers, transformation ratios of n.sup.2 are attainable, where n equals the number of pairs of coaxial conductor sections used in the transformer.
It is also known to construct a high frequency transformer comprising two parallel sections of metal tubing electrically connected at one end and an electrical conductor wound through the hollow interiors of the tubing along their axes. Generally, in such transformers the two tubes form the primary winding with the free ends of the tubes carrying terminals for the input side of the transformer. The electrical conductor which passes an equal number of times through each of the tubes form the secondary winding with the two ends of the conductor carrying terminals for the output side of the transformer. Such transformers provide impedance ratios of n.sup.2, where n represents the number of turns of the conductor through the sections of tubing.
Typical prior art transformers for high frequency transmission lines provide impedance transformation ratios of n.sup.2 :1 (or 1:n.sup.2), where n represents an integer. Such transformers may be adequate for those applications in which the electrical units being coupled have integer-squared ratios. A well known example of such an application is a UHF television antenna network typically having an impedance of 75 ohms and UHF RF signal processing circuitry in the typical television receiver which has an impedance of approximately 300 ohms. Unfortunately, not all electrical devices which are connected by transmission lines have impedances which are integer-squared ratios of each other. In such circumstances, efficient transmission line transformers with non-integer-squared impedance ratios are required.
Accordingly, it is an object of the present invention to provide a novel transmission line transformer with a non-integer-squared impedance transformation ratio.
It is another object of the present invention to provide a novel trannsmission line transformer which minimizers the production of reflected waves.
It is a further object of the present invention to provide a novel transmission line transformer which transforms the characteristic impedance of a transmission line with relatively high efficiency compared to prior transformers.
It is yet another object of the present invention to provide a novel transmission line transformer which may be easily and inexpensively manufactured.
It is a still further object of the present invention to provide a novel transmission line transformer which is capable of operating with transmission lines which are transmitting high voltage, high frequency signals.